1. Field of the Invention
The present invention relates to a signal processing unit applied to a color image device such as an electronic camera, a video camera, a scanner, and a printer, and more particularly, to a signal processing unit for performing color transformation processing or brightness correction processing on each color signal obtained by the color image device.
2. Description of the Related Art
A color transformation circuit is generally mounted on a color image device in order to give a good color reproducibility to a color signal. The color transformation circuit performs color transformation processing on the color signal according to an input/output characteristic peculiar to the color image device.
Note that the word ‘color’ is used in this specification to mean information consisting of hue, brightness, and chroma.
Incidentally, some of the color transformation circuits perform color system transformation of image data as well as color correction.
The color transformation circuits are mainly classified into a lookup table circuit often used in scanners, printers, and the like and a color matrix circuit often used in electronic cameras, video cameras, and the like.
The lookup table circuit stores color signals (R′, G′, B′) which are obtained after color signals (R, G, B) for respective colors are transformed. The lookup table circuit is able to perform not only linear color transformation processing (=first-order color transformation processing) but also nonlinear color transformation processing (=high-order color transformation processing) and is able to set a characteristic curve (color transformation characteristic curve) of the color transformation processing to an optimum one according to the input/output characteristic peculiar to the color image device.
Therefore, the lookup table circuit gives a high color reproducibility to the color signal so that images on a reproduced image are expressed with optimum colors. The lookup table circuit, however, has a disadvantage that it has a large circuit scale due to a large amount of information stored therein.
On the other hand, the color matrix circuit stores a transformation expression (generally, a first-order transformation matrix of 3×3) common to the color signals (R, G, B) for the respective colors. Therefore, the color matrix circuit is able to perform only the linear color transformation processing so that the color transformation characteristic curve can be set to an approximate line, but not to an optimum curve.
Hence, the color matrix circuit cannot give the high color reproducibility to the color signal so that images on a reproduced image are not always expressed with the optimum colors. The color matrix circuit, however, has an advantage that its circuit scale is small owing to small amount of information stored therein and its simple arithmetic operation.
It is possible to set the color transformation characteristic curve to a quadratic curve or a cubic curve by increasing the order of the color transformation processing of the color matrix circuit. The size of the transformation matrix, however, will become ‘3×9’ when the order of the color transformation processing is increased by, for example, one in order to set its color transformation characteristic curve to the quadratic curve. In addition, in this case generation of vectors (R2, G2, B2, RG, GB, BR, R, G, B) increases the necessary number of times of multiplications/divisions by as many as ‘24’ times. Accordingly, to set the color transformation characteristic curve to a high-order curve such as the cubic curve and a quartic curve by using the color matrix circuit, the circuit scale has to be greatly increased.
To solve these problems, a color matrix circuit as disclosed in, for example, Japanese Unexamined Patent Publication Application Nos. Hei 6-6587 and Hei 5-300367 has been conventionally proposed.
In this color matrix circuit, the order of the color transformation processing is ‘1’, but a plurality of element groups of the transformation matrix is prepared in advance. In the color matrix circuit, any of the element groups is selectively used in conformity with to a color range to which a color signal to be transformed belongs. This enables the color transformation characteristic curve to be a stair-shaped line. In other words, in this color matrix circuit the color transformation characteristic curve can approximate to the optimum with reduction in its circuit scale.
According to this color matrix circuit, however, a contour which does not exist in an actual image (a pseudo-contour) and noise may occur on a reproduced image. The reason is as follows.
In general, color change in a certain image is often continuous. If the image belongs to two different color ranges, however, this color matrix circuit performs two different color transformation processings between on a part of the image belonging to one of the two color ranges and on the other part of the image belonging to the other color range. As a result, originally continuous color change in the image becomes discontinuous in the reproduced image.
This problem is not solved even when the number of the element groups prepared in the color matrix circuit is increased. This is because the increase in the number of the element groups increases the number of steps of the color transformation characteristic curve (here, a stair-shaped line), but a level difference between steps still remains.
According to the color matrix circuit in Japanese Unexamined Patent Publication Application No. Hei 6-6587, this level difference among the steps of the color transformation characteristic curve is reduced so that the occurrence of the pseudo-contour can be prevented, but this color matrix circuit may possibly lower the color reproducibility of a certain color image device.
For example, in a color image device using a CCD image sensor for complementary colors, a ‘steeply changing curve’ is the optimum color transformation characteristic curve. In this case, the reduction in the level difference among the steps of the color transformation characteristic curve will result in divergence from the optimum one.